Doesn't seem worth the effort to add it, since overall it somehow has less functionality than what we have now, which is bare bones formatting as far as forums go.
Markdown lacks underline and strikethrough formatting. It also lacks text centering.
Its quote thing is just awful for forums since all it does is indent the stuff that's supposed to be quoted. Honestly I would just say it lacks quoting capability and has indenting, based on how it works.
It lacks youtube embedding code.
Its image embedding code is obtuse. Why  sounded like a good idea to the creator is beyond me, since it utterly violates his own philosophy of being easy to read and write.
Oh, its hyperlinking code appears to be the same way. The code for that is [example], which doesn't make a ton of sense.
It lacks hiders.
What does it have to offer that our current code stuff lacks? Text size alteration, but only in the form of header coding. It sort of allows a font choice through that preformatted code block thing. That's pretty much it, aside from minor convenience in the form of using punctuation marks instead of little code blocks for text formatting.
I don't think that would be a good exchange at all. If there was going to be a change in how the forum text formatting worked, I'd say it would be better to just switch to straight up HTML, which has far more options and is not much more complicated than Markdown or what we have now. Markdown is basically just a set of modifications to HTML (which is what everything is converted into by Markdown anyway), so why bother with the not so great workaround instead of going straight to the source?
It seems to me that this Markdown thing is meant largely for blogging and similar type writing. It just isn't robust enough to bother using on a forum, which is probably why you've never seen it before.
